Lactadherin / MFG-E8

The Structure of Lactadherin
Lactadherin (MFG-E8) binds, Ca++-independently, to phosphatidyl-L-serine via the C2 domain and binds the avb3 and avb5 integrins via the second EGF domain. The one letter amino acid codes refer to identified amino acids that participate in binding. For the C2 domain the capital letters refer to water-exposed hydrophobic residues.

Lactadherin is a widely distributed glycoprotein (~ 50 kDa), which was originally characterized due to its association with milk fat/lipid globule membranes. Synonymous names are PAS-6/7, bovine-associated mucoprotein, BA-46, P47, and MFG-E8. Structural hallmarks of lactadherin are the presence of two epidermal growth factor (EGF) homology domains (with an RGD peptide motif in the second EGF domain), and two C domains sharing homology with the discoidin family of lectin domains including the phospholipid-binding domains of blood clotting factors V and VIII. Lactadherin shows preferential binding to phosphatidylserine (L-form) in a calcium independent manner, and binds more specifically than Annexin V.

Purified lactadherin functions as an anticoagulant by blocking phosphatidylserine-containing membrane sites for blood coagulation proteins (10). Fluoresence-labeled lactadherin functions as a sensitive probe for exposed phosphatidylserine on nucleated cells and on stimulated platelets (8, 9) . Lactadherin will bind to membranes that have phosphatidylserine content below the threshold for annexin V binding.

Above: Phosphatidylserine exposure in mouse mesenteric venous thrombosis. Mice were given 1 µg each of lactadherin and annexin V by tail vein immediately prior to externalization of the mesentery. The mesentary was exposed to ferric chloride and then the animals were perfused with saline/paraformaldehyde. Serial sections of the mesentary were stained with anti-fibrinogen/fibrin (left), anti-platelet (middle), and anti-lactadherin antibodies (right) developed with the alkaline phosphatase Vector Red substrate. A layer of fibrinogen/fibrin (left, closed arrows) overlaid a mural hemorrhage (open star). Platelets (middle) were scattered along the luminal surface of the thrombus (open triangles) as well as upon fibrinogen/fibrin strands extending into the lumen. Lactadherin staining (right) was strongest along the raised endothelium surface (closed arrow), including adherent platelets close to the wall. Platelets on fibrin strands did not stain detectably (open arrow). (Shi J, Pipe SW, Rasmussen JT, Heegaard CW, Gilbert GE. Lactadherin blocks thrombosis and hemostasis in vivo: correlation with platelet phosphatidylserine exposure. J Thromb Haemost. Jul 2008;6(7):1167-1174).